Blood pressure is normally measured by placing a blood pressure cuff around the arm of a patient over the brachial artery. The cuff typically includes an inflatable bladder placed in an outer casing. The bladder is inflated to compress the arm of the patient, thereby pinching off the flow of blood through the brachial artery. The pressure in the bladder is gradually reduced while listening for sounds caused by the flow of blood through the brachial artery and measuring the air pressure in the bladder. When blood flow is detected during systole, the air pressure in the bladder is recorded as the systolic blood pressure. Similarly, when blood flow is detected during diastole, the air pressure in the bladder is recorded as the diastolic blood pressure.
The most common device for measuring blood pressure using the above-described procedure is the familiar manually pumped cuff using a mercury manometer as the pressure measuring device. After the cuff is inflated with air, the pressure in the cuff is gradually reduced while a stethoscope is used to detect the flow of blood in the brachial artery beneath the cuff.
Automated patient monitoring systems are also in common use to allow blood pressure measurements to be periodically taken without the assistance of medical personnel. In these automated systems, a blood pressure cuff is periodically inflated and the blood pressure is then gradually reduced while the cuff pressure is measured. An electrical or acoustic audio pickup device listens for the sound generated as blood starts flowing through the brachial artery to determine the systolic and diastolic blood pressures.
One conventional automated blood pressure monitor utilizes a miniature microphone built into the blood pressure cuff to detect the flow of blood. However, the use of a microphone in the blood pressure cuff has several disadvantages. The microphones, being somewhat fragile, are susceptible to breakage, particularly in heavy clinical use. It is also difficult to maintain the sterility of blood pressure cuffs having built-in microphones. They cannot be sterilized because the heat and moisture of typical autoclaves would destroy the microphone. The common alternative to sterilization in the medical field of making devices disposable after a single use is precluded by the relatively high expense of blood pressure cuffs having build-in microphones. Thus, there is no convenient method of sterilizing blood pressure cuffs having internal microphones. The signal-to-noise ratio of microphones installed in blood pressure cuffs in also limited by the noise picked up in the relatively long leads extending from the cuff to a monitor. Finally, the microphones, being electrical devices, require that the blood pressure cuffs be electrically isolated from patients.
Another approach to combining the blood pressure cuff with a device for detecting blood flow sounds is to use the blood pressure cuff itself as the acoustic pickup device. The pulsating nature of arterial blood flow produces pressure waves that are reflected as pressure variations in the bladder of the cuff. These pressure variations are coupled through a tube conected to the cuff to a pressure transducer where the pressure variations are converted to electrical signals. The use of the bladder as the pickup device has the advantages that it is relatively inexpensive, so it can be disposed of after a single use, and it is very sturdy, so it can be sterilized and can withstand heavy use without damage. Further, because it has no electrical components, electrical isolation from the patient is not required. However, this approach has a major performance limitation because it is capable of picking up only low-frequency blood flow sounds. The pressure waves generated by blood flowing through the brachial artery consist of oscillatory pressure waves having frequency components in the 1 to 6 Hz range and Korotkoff sounds having frequency components in the 18 to 250 Hz range.
It is commonly believed that the auscultatory method of blood pressure monitoring, in which the Korotkoff sounds are used to detect blood flow, is more advantageous than using the oscillatory pressure waves for that purpose. However, a blood pressure cuff bladder is only capable of picking up oscillatory pressure waves; it is not capable of picking up Korotkoff sounds. Basically, the relatively large-volume bladder functions like the electrical equivalent of a capacitor. Just as a capacitor has the tendency to filter or average high-frequency electrical signals, the bladder filters or averages high-frequency acoustic signals so that the high-frequency acoustic signals are not coupled down the tube connected to the bladder. The use of the blood pressure bladder as an acoustic pickup device also has the disadvantage of picking up a great deal of extraneous low-frequency noise that is present in most patient monitoring environments.